1. Field of the Invention
The present invention relates to optical device technology, and more particularly, to a low-loss metal optical waveguide device based on surface plasmons.
2. Discussion of Related Art
A surface plasmon is an oscillating wave of charge density propagating along an interface at which the real number portion of a dielectric constant changes with an opposite sign. In general, a surface plasmon is generated in the interface between a metal having a negative (−) polarity and a dielectric having a positive (+) polarity, and the charge density oscillation may be excited by highly accelerated electrons or light waves. A surface plasmon is a transverse magnetic (TM) polarized wave, and a field having the largest value at a metal surface exponentially decreases with the increase of distance from the metal. Due to this characteristic, a surface plasmon has been frequently used to measure the characteristics of material existing in a metal surface and the optical constant of the metal. In addition, since the field is concentrated near the metal surface, a surface plasmon has many advantages for application in a sensor, etc. Furthermore, a surface plasmon reacts sensitively to the perturbation of a metal surface and thus has application to optical devices using the sensitivity.
In optical waveguides, a surface plasmon obtains a binding condition by a boundary condition between a metal and a dielectric, thus having a drawback in that the propagation length of light is very short, i.e., several tens or hundreds of micrometers (μm), in the visible ray or infrared region. However, when a metal structure is formed into a thin film, the propagation length of a surface plasmon bound in an interface between the metal and the dielectric can be infinitely increased, in theory, using a super mode activated by the interference between surface plasmons propagating along the two interfaces of a thin metal film. Such a long-range surface plasmon was suggested in the early 1980s, and much research on the same has been in progress.
According to a long-range surface plasmon structure in an infinite metal plate, the application of an optical device is possible using optical transmission or field increase. However, a long-range surface plasmon is bound in two dimensions and thus is limited in applicability to an integrated optical device. For this reason, much research has been in progress, which binds a long-range surface plasmon propagating along a metal plate in a direction parallel with the metal surface to be used in integrated optical devices. A surface plasmon is bound in a surface direction by forming a dielectric structure on a metal surface or forming the metal into a structure having a finite cross section. Lately, research on a long-range surface plasmon using a thin metal film having a finite area, i.e., a metal stripe, has been progressing energetically, and also, metal stripe devices have been reported as being produced.
When a metal stripe having a uniform thickness of 10 nm or less is fabricated, a conventional metal stripe optical waveguide using a long-range surface plasmon can theoretically have a propagation loss of 1 dB/cm or less at the communication wavelength of 1.55 μm. However, since there is a limit to fabrication of a uniform metal stripe, realistically it is impossible or very difficult to implement a low-loss waveguide, thereby limiting applicability for various devices.